Switching materials comprising mixed nanoscopic particles and carbon nanotubes and method of making and using the same

ABSTRACT

An improved switching material for forming a composite article over a substrate is disclosed. A first volume of nanotubes is combined with a second volume of nanoscopic particles in a predefined ration relative to the first volume of nanotubes to form a mixture. This mixture can then be deposited over a substrate as a relatively thick composite article via a spin coating process. The composite article may possess improved switching properties over that of a nanotube-only switching article. A method for forming substantially uniform nanoscopic particles of carbon, which contains one or more allotropes of carbon, is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to “Nonvolatile Nanotube Diodes and Nonvolatile Nanotube Blocks and Systems Using Same and Methods of Making Same,” filed on even date herewith (U.S. patent application Ser. No. 12/273,807), which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a spin-coatable material and a method for manufacturing the same for use in the preparation of composite articles and films. More particular, the present invention relates to such a material comprising a first volume of carbon nanotubes and a second volume of nanoscopic particles.

BACKGROUND

Any discussion of the related art throughout this specification should in no way be considered as an admission that such art is widely known or forms part of the common general knowledge in the field.

Nanotube fabric layers (or films) are used within a plurality of semiconductor devices. For example, U.S. patent application Ser. No. 11/835,856 to Bertin et al. teaches methods of using nanotube fabric layers to realize nonvolatile devices such as, but not limited to, block switches, programmable resistive materials, and programmable logic devices.

As described by Bertin et al., a volume of nanotube fabric can be placed into at least two nonvolatile resistive states by passing electric currents through said fabric. These nonvolatile resistive states can be used to create, for example, but not limited to, switch elements which behave as nonvolatile memory cells (wherein, typically, two nominal resistive states are used), nonvolatile variable resistor dividers for precision voltage supplies (wherein, typically, a pair of nanotube devices, each with a plurality of nominal nonvolatile resistive states, are used), and nonvolatile programmable logic devices (wherein, typically, multiple nonvolatile nanotube switch elements are used).

U.S. Pat. No. 7,335,395 to Ward et al. describes methods of applying a nanotube fabric layer on a substrate. Said methods include spin coating (wherein a solution of nanotubes is deposited on a substrate which is then spun to evenly distribute said solution across the surface of said substrate), spray coating (wherein a plurality of nanotube are suspended within an aerosol solution which is then disbursed over a substrate), and in situ growth of nanotube fabric (wherein a thin catalyst layer is first deposited over a substrate and then used to form nanotubes). Further, U.S. Pat. No. 7,375,369 to Sen et al. teaches a nanotube solution which is well suited for forming a nanotube fabric layer over a substrate layer via a spin coating process.

Studies for improved nanotube fabric layer and methods for forming the same are continuing.

SUMMARY OF THE DISCLOSURE

As such, there exists a need for an improved nanotube fabric layer over a substrate and method for manufacturing same. It would be advantageous if said method provided a manner of control over the volume density of nanotubes within said nanotube fabric layer. It would also be advantageous if said method enabled formation of a nanotube fabric layer of significant thickness within a minimum number of spin coat processes, and preferably within a single spin coat process.

The invention provides an improved nanotube fabric layer over a substrate and a method for manufacturing the same.

In particular, the present invention provides a nanotube device comprising a first electrode, a second electrode, and a composite article deposed between said first electrode and said second electrode. The composite article includes a first volume of nanotubes and a second volume of nanoscopic particles in a predefined ratio relative to the first volume of nanotubes.

The nanotube block switch of the present invention is formed over a substrate by first combining a first volume of nanotubes with a second volume of nanoscopic particles in a predefined ratio relative to the first volume of nanotubes to form a mixture material, and thereafter depositing said mixture material over said substrate via a spin coat process. In certain embodiments, the mixture can be homogeneous. In some other embodiments, the mixture can be heterogeneous.

The present invention also provides a method of forming substantially uniform nanoscopic particles of amorphous carbon from a volume of carbon black material. Said method entails reacting, in a first processing step, said volume of carbon black material with an oxidizing agent to form a carbon slurry. Thereafter, in a second processing step, said method entails removing metallic contaminants from said carbon slurry using a solubilization process. Thereafter, in a third processing step, said method entails filtering said carbon slurry to remove solubilized impurities. Thereafter, in a fourth processing step, said method entails increasing the pH level of the carbon slurry to obtain a colloidal system. In certain embodiments, the colloidal system can be homogeneous and/or stable. And thereafter, in a fifth processing step, said method entails further filtering said colloidal system to remove particles which fall above a predetermined volume threshold.

The present invention further provides a resistive material comprising a first volume of nanotubes and a second volume of nanoscopic particles in a predefined ratio relative to the first volume of nanotubes.

The present invention further provides a method for forming a composite article over a substrate is provided. The method can include depositing a first volume of nanotubes over said substrate to form a layer of nanotubes; and depositing a second volume of nanoscopic particles, in a predefined ratio relative to the first volume of nanotubes, on the layer of nanotubes. In certain embodiments, the second volume of nanoscopic particles can be deposited using ion implantation or vapor deposition.

In one aspect of the invention, a first volume of nanotubes is combined with a second volume of nanoscopic particles in a predefined ratio to obtain a mixture. The is can then be applied to a substrate via a spin coat process. The amount of said first volume of nanotubes and said second volume of nanoscopic particles (that is, the ratio of nanotubes to nanoscopic particles within the mixture) are selected such as to provide a desired volume density of nanotubes within the mixture. In this way, a composite article with a desired nanotube volume density can be realized.

In another aspect of the present invention, the nanotubes within the first volume of nanotubes are carbon nanotubes, such as single walled nanotubes.

In another aspect of the present invention, the second volume of nanoscopic particles includes nanoscopic particles which are other allotropes of carbon, including but not limited to, polyaromatic hydrocarbons, graphite, carbon nanopowder, amorphous carbon, carbon black, and diamond.

In another aspect of the present invention, the second volume of nanoscopic particles includes nanoscopic particles which are silicon based materials, including, but not limited to, silicon oxide (SiO₂) and silicon nitride (Si₃N₄).

In another aspect of the present invention, the second volume of nanoscopic particles includes nanoscopic particles which are multi-wall nanotubes (MWNTs).

In another aspect of the present invention, the nanotube volume density within the composite article is optimized to the needs of a given application or device.

Accordingly, it is the object of the present invention to provide an improved method for forming a composite article over a substrate.

It is also an object of the present invention that said method comprise combining a first volume of nanotubes with a second volume of nanoscopic particles in a predefined ratio such as to form a mixture which is well suited for use within a spin coat process.

It is further an object of the present invention that said method provide a method for applying a relatively thick composite article or film over a substrate within a minimum number of spin coat processes and preferably within a single spin coat process.

Other features and advantages of the present invention will become apparent from the following description of the invention which is provided below in relation to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a nanotube device comprising a composite article formed with only carbon nanotubes;

FIG. 2 illustrates a fabrication process suitable for fabricating the nanotube device depicted in FIG. 1;

FIG. 3 illustrates a nanotube device comprising a composite article formed in accordance with certain embodiments of the present invention;

FIG. 4 illustrates a fabrication process for a nanotube device in accordance with certain embodiments of the present invention;

FIG. 5 illustrates a nanotube device in accordance with certain embodiments of the present invention wherein multi-wall nanotubes (MWNTs) are used as nanoscopic particles;

FIG. 6 illustrates a nonvolatile nanotube device in accordance with certain embodiments of the present invention; and

FIG. 7 illustrates the improved switching behavior of the nonvolatile nanotube device of FIG. 6, in accordance with certain embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a conventional nanotube device (e.g., a block switch) which makes use of a nanotube fabric layer comprising only carbon nanotubes. In a first operation, a carbon nanotube fabric layer 130 is deposited over a first electrode 110. The carbon nanotube fabric layer 130 is comprised of a volume of carbon nanotubes 130 a formed into a cohesive fabric or film through the deposition process (e.g., spin coating, spray coating, or in situ growth). In a second operation, a second electrode 120 is deposited over the carbon nanotube fabric layer 130.

In a typical fabrication process, the thickness of the carbon nanotube fabric layer 130 is set above a certain threshold such as to prevent the second electrode 120 from becoming electrically connected (shorted) to the first electrode 110 (via the electrode material bleeding through the carbon nanotube layer 130, for example). As such, a nanotube fabric layer of significant thickness is often required.

FIG. 2 illustrates a conventional fabrication process suitable for realizing the nanotube block switch depicted in FIG. 1.

In a first operation 200 a, a first volume of carbon nanotubes 230 a is deposited over a first electrode element 210 via a spin coating process forming a first intermediate nanotube fabric layer 240 a as illustrated by structure 200 b.

In a second operation 200 c, a second volume of carbon nanotubes 230 b is deposited via a spin coating process over the first intermediate nanotube fabric layer 240 a forming a second intermediate nanotube fabric layer 240 b as illustrated by structure 200 d.

In a third operation 200 e, a third volume of carbon nanotubes is deposited via a spin coating process over the second intermediate nanotube fabric layer 240 b forming a third intermediate nanotube fabric layer 240 c as illustrated by structure 200 f.

In this way, a nanotube fabric layer 240 (the combination of the first, second, and third intermediate nanotube layers 240 a, 240 b, and 240 c) of a required thickness is formed over first electrode element 210. In a fourth operation, a second electrode element 250 is deposited over the nanotube fabric layer 240 as illustrated by structure 200 g.

In its most basic form, certain embodiments of the present invention provide a resistive material that can be used within a plurality of different applications. Such applications include, but are not limited to, display elements, solar panels, and semiconductor circuits. For example, certain embodiments of the present invention include a plurality of nanotube based switching devices, including, but not limited to, block switches, programmable resistive materials, and programmable logic devices.

Furthermore, certain embodiments of the present invention provide resistive materials, including films and fabrics, with controlled and uniform nanotube densities, significantly reducing the cost and/or improving the performance of applications using such materials.

FIG. 3 illustrates one exemplary device, such as a nanotube block switch, in accordance with certain embodiments of the present invention. The device, such as the nanotube block switch shown, includes a composite article 330 containing a mixture of nanotubes 330 a and nanoscopic particles 330 b in a predefined ratio (said nanoscopic particles 330 b depicted as circular elements within FIGS. 3 and 4 for clarity). It should be noted that although FIG. 3 depicts the nanoscopic particles 330 b as the discrete phase and the nanotubes 330 a as the matrix phase, the morphology of the nanoscopic particles 330 b and nanotubes 330 a may be different. For example, in certain embodiments, the nanotubes 330 a may form the discrete phase and the nanoscopic particles 330 b may form the matrix phase. In some other embodiments, the nanotubes 330 a and the nanoscopic particles 330 b can both form interconnected matrix phases. The composite article 330 can act as a switching material between a first electrode 310 and a second electrode 320. The fabrication, function, and use of nanotube block switches is discussed in greater detail within U.S. patent application Ser. No. 11/835,856 to Bertin et al., which is incorporated by reference herein in its entirety.

The nanoscopic particles are purposefully introduced in a predefined ratio with respect to the nanotubes to control the composition and, consequently, physical, electrical, and thermal aspects of the resulting composite articles. Whereas in other contexts, nanoscopic particles other than nanotubes might be viewed as undesirable impurities, in the composite article of the present invention, the nanoscopic particles are a deliberately added component, introduced to achieve the desired device performance, such as desired switching attributes. Indeed, the nanoscopic particles are selectively mixed with nanotubes to form a composite article having a predefined volumetric ratio of nanoscopic particles to nanotubes. The ratio may be pre-selected and tuned to ensure, for example, the desired range of electrical switching or resistive states. The attributes of the nanoscopic particles—the material, the size, the uniformity of the particulate population, the shape of the nanoscopic particles, its interaction with the nanotubes, etc.—can all be specifically selected to further tune the desired device characteristics (e.g., electrical switching or resistive characteristics) of the resultant composite article. Moreover, in certain instances, the attributes of the nanoscopic particles itself may further dictate the predefined ratio of the nanoscopic particles and nanotubes. Regardless, in each case, the purposeful and deliberate addition of nanoscopic particles can have the common effect of allowing inventors additional control in tuning and refining the characteristics (electrical, physical, thermal or otherwise) of the composite article. For example, addition of the nanoscopic particles in a predefined ratio with the nanotubes may decrease the switching voltages of the composite article as compared to switches formed from pristine nanotubes.

The predefined ratio of the nanoscopic particles to the nanotubes can be any ratio selected by the manufacturer depending on the application, method of combination, or the composition of materials used in the device. For example, in certain applications, some suitable and non-limiting predefined ratio of the nanoscopic particles to the nanotubes may be from about 1:1 (one part nanoscopic particles to about one part nanotubes) to about 1:10 (one part nanoscopic particles to about ten part nanotubes). For example, some suitable and non-limiting predefined ratio of the nanoscopic particles to the nanotubes may be 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10.

The nanoscopic particles 330 b can take a plurality of forms depending on the needs of an application or structure in which the methods of the present invention are employed. The nanoscopic particles may be spherical, oblong, square, irregular, or any other shapes as would be readily apparent to ordinary skill in the art. The nanoscopic particles may have at least one dimension that is in the nanometer size. For example, the nanoscopic particles may have at least one dimension which is less than 100 nm, 50 nm, 40 nm, 30 nm, 25 nm, 20 nm, 10 nm, 5 nm, or 1 nm. In certain embodiments, the nanoscopic particles may have dimensions that are acceptable in semiconductor fabrication facilities, such as a CMOS facility. In certain embodiments, the nanoscopic particles may be individual atoms or ions.

The nanoscopic particle can interact covalently or non-covalently to another nanoscopic material, for example, carbon nanotubes. In certain embodiments, the nanoscopic particles may be miscible with the nanotubes and form a continuous material around the nanotube. In some other embodiments, the nanoscopic particles may be inert to the nanotubes and remain in the same form as initially introduced into the mixture and therefore non-miscible. In yet some other embodiments, the nanoscopic particles may be partially miscible with the nanotubes and form a semi-miscible mixture with the nanotubes. In certain embodiments, the nanoscopic particles may have the ability to alter the porosity between the carbon nanotubes.

The nanscopic particles may be introduced to the composite article either before deposition on the substrate or after the nanotube is applied to the substrate. In the first application, the nanoscopic particles can be combined with the carbon nanotubes by introducing them into the solution containing carbon nanotubes then depositing the combined mixture onto the substrate. In the second application, the nanoscopic particles can be introduced, for example, by ion implantation, vapor deposition, sputtering, or other methods known in the art after first forming a nanotube layer on the substrate.

Furthermore, in certain embodiments, the choice of such nanoscopic particles can include a material or materials that can be formed with a uniform particle size. In certain applications, the choice of a nanoscopic particle can include a material or materials which can be fabricated as individual particles within certain dimensions. For example, an application may require a nanoscopic particle wherein individual particles are not larger than some fraction of a device feature size.

In some other embodiments, the choice of such nanoscopic particles can include a material or materials which do not adversely affect the switching operation (that is, the changing from one nominal nonvolatile resistive state to another) of the composite article. In fact, in certain embodiments, the nanoscopic particles 330 b may improve switching operation by lowering the voltage needed for the composite article to change its resistance.

In some other embodiments, inorganic nanoparticles can be utilized. For example, silicon based materials (such as, but not limited to silicon oxide and silicon nitride) can be used for said nanoscopic particles 330 b.

In some embodiments, one or more allotropes of carbon (such as, but not limited to, diamond, graphite, graphene, fullerenes, amorphous carbon, carbon black, carbon nanopowder, carbon nanobuds, carbon nanorods, carbon nanofoam, lonsdaleite, linear acetylenic carbon, polyaromatic hydrocarbons, and the like) can be used for said nanoscopic particles 330 b.

In certain embodiments, nanoscopic particles 330 b can include a mixture of different nanoscopic materials, such as any combination of nanoscopic particles 330 b described above.

The nanoscopic particles 330 b can be obtained by numerous different ways. For example, carbon particles having of particles of substantially uniform volume can be obtained through the process described below. Methods for obtaining other desired nanoscopic materials 330 b will be readily apparent to one of ordinary skilled in the art.

-   -   In a first processing step, reacting a volume of carbon black         material with an oxidizing agent (such as, but not limited to,         nitric acid) to form a carbon slurry in order to decrease the         size of carbon black particles and further remove any metallic         contaminants (via solubilization). The first processing step may         be aided by further introducing other acids, such as         hydrochloric acid.     -   In next processing step, filtering the carbon slurry formed in         the first process step at low pH (for example, but not limited         to, via cross-flow membranes) to remove any solubilized         impurities     -   In a next processing step, increasing pH level of the carbon         slurry to realize a homogeneous and stable colloidal system (in         some operations, a sonication process may be used to improve         homogeneity)     -   In a next processing step, filtering the realized homogeneous         and stable colloidal system through a train of filters to remove         any particles which could lead to defects in the spin coated         film (in some operations, for example, said system would be         passed through filters with pores as small as 10 nm or 5 nm or         other filters with the smallest pore size available)

As described in greater detail below, the resulting colloidal system of processed carbon particles can then be combined with a carbon nanotube solution at a ratio which will enable the generation of a film or fabric layer which will comprise a desired volume density of carbon nanotubes.

FIG. 4 illustrates a nanotube block switch fabrication process in accordance with certain embodiments of the present invention. A first volume 420 of nanotubes 420 a is combined with a second volume 410 of nanoscopic particles 410 a to obtain a mixture 430. The mixture 430 can be homogeneous or heterogeneous. One of ordinary skill in the art will readily appreciate the various different ways the mixture 430 can be formed.

In certain embodiments, mixture 430 can be formed so that the mixture 430 can be utilized in semiconductor fabrication facilities, such as in Class 100, 10, or 1 facilities (e.g., CMOS facilities). For example, the mixture 430 can be substantially free of undesirable particulate and metal impurities, such as being substantially free of particulate impurities that are greater than 1000 nm, 500 nm, 400 nm, or even 300 nm in diameter. As another example, the nanotubes 420 a and nanoscopic particles 410 a can be combined in a solvent that is acceptable for use in semiconductor facilities, such as an aqueous (e.g., highly purified water) or non-aqueous solvents that are compatible with semiconductor fabrication processes.

In process step 400 a, the mixture 430 is deposited over a first electrode element 440 via a spin coating process to form composite article 450 (as illustrated by structure 400 b). The mixture 430 allows for the deposition of significantly thicker (as compared to prior art nanotube solutions) layers (or films) within a single spin coat process as compared to nanotube-only liquids. For example, thickness ranging from about a few to hundreds of nanometers may be possible through a single coat. Some non-limiting example thicknesses that can be achieve include 1, 2, 2.5, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, or 200 nm. As such, a sufficiently thick composite article suitable for use within a plurality of nanotube block switching devices (such as, but not limited to, block switches, programmable resistive materials, and programmable logic devices) can be realized in a minimum number of spin coat process steps. Further, in many applications, such a layer can be realized within a single spin coat process step, significantly reducing process time and cost.

Then, a second electrode element 460 is deposited over the composite article 450 as illustrated by structure 400 c.

FIG. 5 illustrates a nanotube block switch with a composite article 530 formed via an alternate embodiment of the methods of the present invention. A first volume of single wall nanotubes (SWNTs) 530 a can be combined with a second volume of multi-wall nanotubes (MWNTs) 530 b to form a mixture. In certain embodiments, said second volume of MWNTs 530 b can act as the nanoscopic particles while said first volume of SWNTs 530 a are single walled carbon nanotubes.

Then, said mixture can be deposited over a first electrode 510 via a spin coating process to form composite article 530. The composite article 530 can have very low level metal contamination. For example, the composite article 530 may have less than 10¹⁸, 10¹⁶, 10¹⁵, 10¹⁴, 10¹³, 10¹², 5×10¹¹, 1×10¹¹, 5×10¹⁰, or even less than 1×10¹⁰ atoms/cm². Thereafter, a second electrode 520 can be deposited over the composite article 530.

It should be noted that while FIGS. 4 and 5 depict two exemplary fabrication process in order to clearly illustrate the methods of the present invention, said methods are not limited to these exemplary embodiments. Rather, one of ordinary skill in the art would readily recognize other methods for forming the desired nanotube devices. For example, the methods of the present invention are well suited to forming a relatively thick carbon nanotube films. However, the present invention is not limited in this regard. Indeed, the methods of the present invention are applicable to a plurality of applications wherein a specific volume density of carbon nanotubes is required within a carbon nanotube film, including, but not limited to, those applications employing very thin films. Accordingly, one of ordinary skill in the art would readily recognize the various different embodiments for fabrication the desired nanotube devices of interest.

EXAMPLE

FIG. 6 depicts a nonvolatile nanotube switch. As shown, cell structure 8700 having cell 8705 comprises a composite article 8710 containing nanotubes and nanoscopic particles. The nanoscopic particles can include carbon particles having substantially uniform volume as described above. The composite article 8750 has top/end contact 8765 and bottom 8730 contact. The present cell select and control structure includes conductive plug 8710 connecting bottom contact 8730 to an N+ region embedded in P-type substrate PSUB. In the present cross sectional view word line WL1 is used as one portion of the cell select circuitry. Cell 8705 may be integrated on a 1024 bit array for the purposes of electrical testing to evaluate electrical characteristics of the composite article 8750. In one or more embodiments, tests include SET to program the cell (write 1), RESET to erase the cell (write 0) and READ to access the stored state of the cell. SET, RESET and READ functions are known in the art and discussed in greater detail above in relation to 3-D cell structures employing nanotube articles.

FIG. 7 summarizes typical RESET and SET electrical parameters, according to one or more embodiments. Specifically, typical applied pulse rise and fall times, duration, voltages and currents are listed. Testing has revealed that in certain embodiments and switch structures, the composite article 8710 containing nanotubes and nanoscopic particles enables a lower operating voltage than does the CNT-only material counterpart. For example, various embodiments of the nonvolatile nanotube switch functions at operating voltages less than or equal to approximately 5.0V. As a point of comparison, various switching structures having CNT-only materials to form the carbon nanotube articles typically function at operating voltages between approximately 7.0 and 8.0V. Moreover, testing has suggested that the composite article 8710 containing nanotubes and nanoscopic particles, when used in certain switch configurations, may be faster in performing the SET function than a CNT-only material counterpart. In other words, the composite article 8710 containing nanotubes and nanoscopic particles may, in certain embodiments, be programmable under shorter duration write 1 operations.

Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention not be limited by the specific disclosure herein. 

1. A two-terminal switching device comprising: a first electrode; a second electrode; and a switching composite article disposed between and in constant electrical communication with each of said first electrode and said second electrode of said two terminal switching device, wherein said composite article comprises a first volume of nanotubes and a second volume of nanoscopic particles in a predefined ratio relative to said first volume of nanotubes.
 2. The two-terminal switching device of claim 1 wherein the nanotubes comprising said first volume are substantially all carbon nanotubes.
 3. The two-terminal switching device of claim 1 wherein said second volume of nanoscopic particles includes nanoscopic particles containing at least one allotrope of carbon.
 4. The two-terminal switching device of claim 1 wherein said second volume of nanoscopic particles includes silicon oxide particles.
 5. The two-terminal switching device of claim 1 wherein said second volume of nanoscopic particles includes silicon nitride particles.
 6. The two-terminal switching device of claim 1 wherein said second volume of nanoscopic particles includes multi-walled nanotubes.
 7. The two-terminal switching device of claim 1 wherein substantially all of said nanoscopic particles within said second volume fall within a predetermined volume tolerance.
 8. The two-terminal switching device of claim 1 wherein at least one of the magnitude of said first volume of nanotubes and the magnitude of said second volume of nanoscopic particles is selected to realize a desired volume density of nanotubes within said switching composite article.
 9. The two-terminal switching device of claim 1, wherein said first volume of nanotubes form a matrix phase and said second volume of nanoscopic particles form a discrete phase.
 10. The two-terminal switching device of claim 1, wherein said second volume of nanoscopic particles form a matrix phase and said first volume of nanotubes form a discrete phase.
 11. The two-terminal switching device of claim 1, wherein said first volume of nanotubes and said second volume of nanoscopic particles form interconnected matrix phases.
 12. The two-terminal switching device of claim 1, wherein said composite article has less than about 10¹¹ metal atoms/cm². 